Why Repeated Distillation Cannot Produce Pure Ethanol
A small distillery ferments an 8.0% ethanol–water wash and plans to recover pure ethanol by repeated simple distillation. The operator believes each pass multiplies the ethanol concentration. The still is heated with a heating mantle, the vapour is condensed, and the first distillate is collected at 78–80 °C. After three passes the product assays 95.6% ethanol by mass and boils at 78.2 °C; further distillation at atmospheric pressure does not change the composition. The operator asks whether a taller fractionating column or a lower operating pressure will solve the problem. The laboratory has anhydrous calcium oxide, molecular sieve 3A, and a rotary evaporator. Safety data: ethanol vapour is flammable above 13 °C; the still has no explosion-proof wiring, and the condenser water supply is intermittent.
1. Explain why repeated distillation cannot exceed 95.6% ethanol, using intermolecular forces.
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Ethanol and water form a minimum-boiling azeotrope at about 95.6% ethanol by mass and 78.2 °C. The mixture deviates positively from Raoult's law: the ethanol–water hydrogen bonds are weaker than the ethanol–ethanol and water–water interactions, so the vapour pressure is higher than ideal and the boiling point is lower. At the azeotrope the vapour and liquid have the same composition, so each distillation step approaches but cannot pass that composition. A taller column increases the number of theoretical plates and reaches the azeotrope more quickly, but it cannot break the azeotropic point. The observation that further distillation gives no change is the operational signature of the azeotrope.
2. Evaluate the two proposals: a taller fractionating column and a lower operating pressure. State whether either gives pure ethanol in one stage, and explain.
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A taller column does not give pure ethanol; it only improves separation up to the azeotrope. Lower pressure changes the azeotrope composition and boiling temperature because the activity coefficients depend on pressure; pressure-swing distillation uses this to cross the azeotrope, but it requires two columns at different pressures and a control system. A single lower-pressure still will still hit an azeotrope, and the intermittent condenser water supply makes vacuum operation unsafe. So neither proposal alone gives pure ethanol.
3. Recommend a safe method to obtain dry ethanol from the 95.6% product, with a chemical or physical reason. Include a safety judgement.
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Treat the 95.6% product with freshly ignited calcium oxide: CaO + H2O → Ca(OH)2 removes water, and then distil the ethanol. Molecular sieve 3A is an alternative: its pores admit water (kinetic diameter about 0.27 nm) but exclude ethanol (about 0.44 nm), so it adsorbs water. A rotary evaporator will not dry ethanol; it removes solvent. Safety: carry out the distillation with an explosion-proof heater or a heating mantle with no exposed element, ensure the condenser water is reliable, and ground the apparatus; ethanol vapour is flammable above 13 °C. Do not use sodium metal because it reacts violently with any residual water.